Depression in the medically ill occurs 5-10 times more often than depression in the general population and has a significant impact on treatment adherence, quality of life, and morbidity and mortality. New developments in the conceptualization of depression in the medically ill have focused on the potential role of immune activation/inflammation and the associated release of proinflammatory cytokines. Proinflammatory cytokines have been found to influence neurobiologic function and induce a depressive syndrome that has overlapping features with major depression. The long term objective of the proposed work is to further understand the pathophysiology and treatment of this cytokine-induced depression as it relates to depression in the medically ill. To accomplish this goal, we plan to develop an animal model of cytokine-induced depression using rhesus monkeys administered the cytokine, interferon (IFN) alpha. IFN alpha is a potent inducer of proinflammatory cytokines (especially interleukin 6) and leads to depressive symptoms in 30-50% of patients depending on dose. In addition, IFN alpha activates corticotropin releasing factor (CRF) pathways and has been shown to lead to monoamine depletion in laboratory animals (including rhesus monkeys). In addition, IFN alpha has been shown to alter fronto-striatal neurocircuitry in humans and induce REM sleep changes (decreased REM latency, increased REM percentage) consistent with depression in both humans and rhesus animals. Thus, IFN alpha treatment provides a unique model system to further understand the pathophysiology and treatment of cytokine-induced mood disorders. The specific aims of the proposed work are 1) to characterize neuroendocrine, monoamine, immune and behavioral responses of rhesus monkeys to IFN alpha and 2) to examine the therapeutic efficacy (capacity to reverse IFN alpha-induced neuroendocrine, monoamine, immune and behavioral changes) of pharmacologic compounds that antagonize activation of the cytokine network (NK-1 antagonists), antagonize CRF, or increase the activity of neurotransmission in monoamine neurocircuits. Relevant techniques to be used to accomplish these aims will include repeated blood and CSF sampling, telemetric polysomnography, microPET, and behavioral analysis of fear potentiated startle and social interactions. Results from these studies will identify novel targets as well as pharmacologic strategies for treatment of mood disorders in the medically ill.